1. Field of the Invention
The present invention relates to a toner density detecting device for detecting the density of a toner and a toner supply device for supplying the toner for use in electronic photographic device and the like.
2. Description of the Prior Art
In general, an electronic photographic device utilizes a two-component developer consisting of a mixture of toner and carrier. It is well known that the weight mixing ratio of the toner to the carrier in the developer is an extremely important factor in order to improve the developing efficiency. If the weight ratio of the toner to the developer (hereinafter referred to as the toner density) is low, that is, if the density of the developer is reduced, the density of a resultant image will become thin on the other hand, if the toner density is too high, such inconveniences will occur that the density of the resultant image become too thick and fog is produced. Therefore, the continuous developing of images of desired tone quality requires adequate control of the toner density and keep the thus controlled density constant.
Conventionally, two techniques for optically measuring the toner density have been known.
In a first technique, a toner density detecting area is provided on a light sensitive drum and a toner is caused to adhere to the drum, thereby measuring the reflected ray indicative of the toner density (reflected density). However, this method has such drawbacks as follows:
(1) Since the toner density detecting area must be provided on the light sensitive drum, the operating sequence is complicated.
(2) Since the light receiving element and light source for use in detection tend to become stained with splashed toner, a separate cleaning device is needed.
(3) The detection level becomes unstable due to variations in latent image potential occurring on the light sensitive drum.
In a second technique, a bias voltage is directly applied to a conductive member and the toner is caused to adhere to the conductive member, thereby measuring the reflected or transmitted ray indicative of the toner density. However, this method also suffers from such problems as follows:
(1) As the above-mentioned bias voltage is increased, breakdown occurs in the developer at a relatively low voltage. Therefore, the density must be measured at a low bias voltage, so that an accurate and stable density measurement is impossible, owing to the fact that since the density of the developer does not reach a saturated state at a low, bias voltage in its V(voltage) - D(density) characteristic, the valve of density is drastically varied with a slight variation in bias voltage. That is, the curve of the above-mentioned V - D characteristic is so steep that the density at a specific bias voltage can not be accurately detected.
(2) If the density is measured in an unsaturated area in the V - D characteristic, a highly accurate bias voltage source is needed and hence the measurement involves great expense.
In addition, as another toner density measuring method, a method of measuring the toner density utilizing a difference in physical property between the toner and the carrier, such as a difference in magnetic permeability, spectrum reflection characteristic, electric conductivity, flow resistance, dielectric constant and the like, is known.
However, all of the above-mentioned methods have circumstance-depending characteristics. In particular, when the brush resistance of a developer is lowered at a high humidity, the developer exhibits a property quite different from that observed at normal humidity, so that the detection level of the toner density is changed and hence a great amount of toner is continuously supplied, which inconveniently causes an excess in toner density and a foggy white background.